The present invention relates to a balancing device for a raisable-curtain goods-handling door: doors in factories, warehouses, garages, shops, etc., comprise a shaft for winding up a curtain or for winding up straps that raise a curtain, said shaft having two drums for winding belts on which two counterweights are suspended, the curtain being intended mainly for opening or closing a door-bay which is generally provided to allow vehicles to pass therethrough, e.g. cars, trucks, goods-handling vehicles, etc.
Such a door is usually actuated by an electric motor.
In order to reduce the motor driving force, and also to make it possible to operate the door in the event of a power failure, provision is made for a counterweight to move down while the curtain door moves up. Although effective in numerous applications, there are cases where drawbacks arise. In particular, with some doors, the weight of the hanging portion of the curtain varies, becoming less as the curtain moves up.
For example, when the curtain is wound onto a shaft at the top of the curtain, the wound-up portion of the curtain no longer exerts a gravity force, and only the hanging portion that has not yet been wound up needs to be supported. The curtain may be constituted by panels that slide between slideways, with the panels being folded horizontally or nearly horizontally above the door, such that panels disposed on a horizontal slideway portion no longer exert any gravity force. There also exists doors made up of panels which on being raised are stored above the door in a position where they no longer exert a gravity force. Other such situations also exist. The invention also applies to doors in sections where door elements are tilted into a horizontal position, providing the door-raising mechanism is in driving connection with a shaft for winding counterweight belts.
Whatever the structure of the curtain, when it comes to the top of its upward stroke, in the above cases, it exerts relatively little residual gravity force whereas the force exerted by the counterweights does not vary. A damaging inertia effect can thus occur at the end of the upward stroke.
Proposals have already been made for a counterweight device proposing a plurality of masses which stop at different levels as they move downwards (see U.S. Pat. No. 1,750,042 and Soviet patent 661.098).
That device is effective, but is not suitable for all applications.